My four-year-old PowerBook G4 is putting itself into sleep mode and refusing to wake up. It gives a very unfriendly beep and a black screen when it is turned on. Taking out and replacing the memory will eventually bring it to life.Veronica Piekosz

For a first attempt, try resetting the Mac's PRAM and NVRAM and its power management unit (PMU). If that doesn't work, I'd suspect a hardware fault, though that might be as simple as a flat battery. Some early models of the PowerBook G4 had a reed switch that tended to fail, and another common problem was memory slot failures. Apple extended the warranty for machines with memory slot failures, but in the US, there's a lawsuit over machines with serial numbers outside the extended warranty scheme that failed anyway.

However, the G4 has an old PowerPC chip, and is obsolete because Apple has long since moved to Intel chips. Whether it's worth repairing depends on how much that costs, and whether you are able to splash out on a new one. The cheapest new MacBook with 2GB of memory and a 120GB hard drive is £769* direct from Apple UK, post free; but it only has a 13.3 inch screen, not the G4's 15-inch screen. The cheapest model with a 15.4 inch screen is a MacBook Pro at £1,399.

* £719 with only 1GB of memory. The published version has a small editing mistake.

Backchat 1: Veronica has now taken it to an independent repairer, and she says: ""It looks very much like I've got this memory slot problem, which requires a new -- expensive -- logic board to fix. I bought the machine in 2005, but the extended warranty expired in July this year. Apple have washed their hands of it, despite me arguing that they sold me a machine with a known problem, ie not of merchantable quality."

Backchat 2: In my answer, I mentioned that in the US, Apple had extended the warranty for models with a known memory slot fault. Veronica found out that her machine's serial number was in the range affected, and her Trading Standards office said she seemed to have a good case to take to the small claims court. She says: "I called Apple again, and this time they agreed to pay for the new logic board (£600!) needed to fix the problem."